Sound is kinetic energy released by vibrations of molecules in a medium, such as air. In industrial applications, sound may be generated in any number of ways or in response to any number of events. For example, sound may be generated in response to vibrations resulting from impacts or frictional contact between two or more bodies. Sound may also be generated in response to vibrations resulting from the rotation of one or more bodies such as shafts, e.g., by motors or other prime movers. Sound may be further generated in response to vibrations caused by fluid flow over one or more bodies. In essence, any movement of molecules, or contact between molecules, that causes a vibration may result in the emission of sound at a pressure level or intensity, and at one or more frequencies.
The use of unmanned aerial vehicles such as airplanes or helicopters having one or more propellers is increasingly common. Such vehicles may include fixed-wing aircraft, or rotary wing aircraft such as quad-copters (e.g., a helicopter having four rotatable propellers), octo-copters (e.g., a helicopter having eight rotatable propellers) or other vertical take-off and landing (or VTOL) aircraft having one or more propellers. Typically, each of the propellers is powered by one or more rotating motors or other prime movers.
A propeller is statically balanced (or in static balance) when the propeller remains at rest, and may remain in any position, when the propeller is not powered. A propeller is dynamically balanced (or in dynamic balance) when the propeller rotates evenly and without vibration. For example, a propeller that is statically balanced may be dynamically imbalanced, when the blades of the propeller have different centers of mass or gravity, or centers of mass or gravity that are not in common planes, such that centrifugal forces act on the blades in different planes and do not counteract one another. Conversely, where a propeller is dynamically balanced, centrifugal forces acting on the blades are equal to and counteract one another, and any vibrations observed should be minimal.
Traditionally, the balancing of propellers has been recognized one of the most important considerations of a properly operating aerial vehicle. For example, in aircraft having large propellers, vibrations generated by propellers that are either statically or dynamically imbalanced have resulted in undue stresses to crankshafts or other component parts. In aerial vehicles of all sizes, such vibrations may result in undesired or untenable noise levels within a vicinity of the rotating aircraft.
An aerial vehicle is typically outfitted with a homogenous set of propellers that are balanced, both statically and dynamically, during operation. The propellers may be operated collectively or in groups. For example, a quad-copter having four propellers may operate each of the four propellers during take-off or landing evolutions, where the quad-copter's lift capacity is preferably maximized. When the quad-copter is aloft at a desired altitude, and a maximum lift capacity is no longer desired, motors associated with one or more of the propellers may be stopped for any reason, such as to preserve power or fuel.